Abstract

Seismic sections A-A' and B-B' cut across a left lateral wrench fault. The profiles are located by gray bars in Figure 1 which is a map of faults cutting seismic reflector R3. The wrench fault parallels an older Late Cretaceous arc-trench system. In middle Miocene strike-slip movements occurred on old fault planes in the basement melange (below R5 on the seismic sections) and initiated formation of a wrench fault system in the overlying sedimentary cover. Fault movements continued intermittently to the present.

In Figure 1, the heavy black line marks the trace of the major wrench fault D. The thin lines are subsidiary cults genetically related to D. The most prominent subsidiary faults are the Riedel shears, R. Riedel shears are oriented 10 to 20° counterclockwise from the main fault D and have the same sense of strike-slip offset as D. Field studies and laboratory experiments indicate that Riedel shears form 10 to 20° clockwise from D and also have the same displacement sense. P shears connect en echelon Riedel shears to form diamond-shaped horst and graben blocks that are characteristic of mature wrench fault zones. A horst block of this type is shaded gray in Figure 1. Extension faults, E, are oriented approximately 45° counterclockwise from the major wrench fault and have normal dip-slip separations.

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